I am interested to remote control something (sorry for the vagueness) over a distance of several hundred kilometers to a place where there are no internet cables. Therefore, I am interested to know how the US Military remote controlls their drones. My best guess is via satelite communicatation, but wouldn't there be too much lag to make this a feasible option, at least when considering the speed these drones fly with? I also wonder whether such satelite communication is available for civil purposes at a reasonably affordable price?
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\$\begingroup\$ Sure there's lag in satellite control, but it's not like there needs to be sub-second responses when flying through a tremendous volume of essentially empty space. A Boeing 767 doesn't need to maneuver on a dime just because it's fast. \$\endgroup\$– Nick TApr 27, 2011 at 16:33
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\$\begingroup\$ @Nick T I would not feel confident to push "fire" on a laster controlled rocket, if there was one second of delay, unless the underlaying systems had a really nice way of handling the latency. \$\endgroup\$– DavidApr 27, 2011 at 16:38
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\$\begingroup\$ Also, these are military satellites they're using, not consumer grade. They may have significantly less latency due due to lower load, priority for certain devices, or better/newer hardware. \$\endgroup\$– pfyonApr 27, 2011 at 17:03
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2\$\begingroup\$ The "latency" in a rocket or bomb reaching it's destination after firing is going to swamp any latency in the communication systems. \$\endgroup\$– Nick TApr 27, 2011 at 18:17
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2\$\begingroup\$ The latency is mainly determined by the distance. If the satellites are in a low orbit the latency will be low low enough, while with satellites in geostationary orbit it could be a problem. \$\endgroup\$– starblueApr 27, 2011 at 21:06
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3 Answers
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Try checking out http://www.diydrones.com/ it may answer a lot of your questions.
As for multi-km links RF waves travel at the speed of light which is 299,792,458 meters per second that's more than fast enough for realtime control. The problem with long distance links is that for a frequency high enough to support realtime high-quality video such as microwave frequencies are line of sight for the most part.
In order to get past free space attenuation you'll also need some power over FCC legal limits I'd assume, but you can get past that by getting an amateur radio license. There are ways around this, directional antennas but thats a whole story by itself. (I am not a lawyer, I am however an Amateur Radio Operator)
Free space attenuation of signals (path loss) can be calculated as follows: Lfs = 32.45 + 20 log d + 20 log f
Where
Lfs = Free space loss in dB
d = distance in km
f = frequency in Mhz
Hope this helps!
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Microhard http://www.microhardcorp.com/ make nice tiny, light 900Mhz (ISM band) radio modules that bridge ethernet (and serial and USB)
- 1.5Mbps over 90-100km without a repeater.
- Add a repeater every 90-100km, or put the repeater on a tall hill every 300km
- The export restricted versions do 128 bit encryption as well.
- We use the unrestricted module on our robots, which are a lot like US military drones.
- Our old military UAV had an older Microhard radio module (lower bit rates).
- I'm pretty sure most of the short range ( 1-200km) stuff is using the 900Mhz band.
- The module the size of a big matchbox and lightweight.
Why use the 900Mhz ISM band?
- Because it works well, even in built up areas.
- Doesn't need a special radio license to use.
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I remember reading an article about how the older drones used an unencrypted wireless signal (apparently they've all now been retrofitted with newer hardware that can support real-time encryption of video).
My guess is they just use a high powered transmitter to send a signal to a base station that can then relay the data with a more traditional network anywhere they need it.
Unfortunately, I don't know where I read this article.
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\$\begingroup\$ I would be very interested to know their range over enemy territory, then. It should be quite limited. \$\endgroup\$– DavidApr 27, 2011 at 16:26
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3\$\begingroup\$ The surveillance video is unencrypted, mostly for the convenience of the ground troops using it, while the command and control channel is encrypted. schneier.com/blog/archives/2009/12/intercepting_pr.html \$\endgroup\$– starblueApr 27, 2011 at 21:11